1. Field of the Invention
The present application related generally to semiconductor detector devices, such as a PIN diodes or a silicon drift detectors.
2. Related Art
Prior art PIN diodes 110, such as is shown in FIG. 11, can include an intrinsic region or substrate 11, which can have a first conduction type, disposed between a top region 12, having the first conduction type, and an entrance window 115, having a second conduction type. The top region 12 can be substantially more highly doped than the substrate 11. The first conduction type is typically n type and the second conduction type is typically p type. A voltage V11 can be applied to the top region 12 and another voltage V12 can be applied to the entrance window.
For example, the top region 12 can be n-doped and can have a voltage of about 100 volts. The entrance window 115 can be p-doped and can have a voltage of about 0 volts. An x-ray photon can be absorbed in the substrate 11, resulting in production of electron-hole pairs. The number of electron-hole pairs produced is directly proportional to the energy of the x-ray photon and provides information about the type of material or process that produced the x-ray photon.
The free electrons can be drawn to the top region 12 and the holes can be drawn to the entrance window 115 due to the voltage differential between the entrance window 115 and the top region 12. The motion of the electrons and holes can generate a signal whose size is proportional to the number of electrons. Based on the size of this signal, the energy of the x-ray photon, and thus the type of material or physical process that produced the x-ray photon, may be determined.
Some x-ray photons can be absorbed in the entrance window 115 and produce electron-hole pairs in the entrance window. Such electrons typically recombine with holes and do not move through the substrate 11. No signal results from such x-ray photons. Thus it can be beneficial to keep the entrance window 115 thin to avoid uncounted electron-hole pairs resulting from x-ray photons being absorbed in the entrance window 115.
Entrance windows can be made by epitaxy deposition, diffusion, or implantation, such as p-doping a substrate, for example with boron. A junction layer 116 can exist between the entrance window 115 and the substrate 11 in which there is a gradual change in concentration of dopant atoms. For example, if the entrance window is p doped and the substrate is n doped, then the junction layer 116 can have a gradual reduction of p dopants, from a high concentration near the entrance window 115 to a lower concentration of p dopants and eventually no p dopants moving farther from the entrance window 115 into the substrate 11. X-ray photons absorbed in the junction layer 116 can also cause production of electron hole pairs. For a single x-ray photon being absorbed in or near the junction layer 116, sometimes only a fraction of the electrons produced flow through the substrate 11 and reach the top region 12 while other electrons produced will recombine with holes. This results in only a partial electron signal and contributes to background signal. Background signal can cause difficulty in distinguishing elements that are of low percent in a sample being analyzed because signals from such low percentage elements can be indistinguishable from the background signal. Therefore it can be beneficial to reduce the thickness of this junction layer 116 in order to reduce the background signal.
Prior art silicon drift detectors 120, such as is shown in FIG. 12, operate similar to PIN diodes with an intrinsic region or substrate 11 with a first conduction type, a top island region 22 having the first conduction type disposed at one surface and an entrance window 115 having the second conduction type disposed at an opposing surface. A difference between silicon drift detectors 120 and PIN diodes 110 is the addition of doped rings 125 having the second conduction type surrounding the top island region 22. The rings can be electrically coupled such as by a MOSFET structure or resistor chain 127. A voltage V13 can be applied to an inner doped ring 125a and a different voltage V14 can be applied to an outer doped ring 125b. This voltage differential can aid in drawing electrons towards the top island region 22. For example, a voltage V12 on the entrance window 115 can be −100 volts, a voltage V14 on an outer ring 125b can be −160 volts, a voltage V13 on an inner ring 125a can be −20 volts, and a voltage V11 on the top island region 22 can be about 0 volts. Silicon drift detectors can suffer from the same entrance window problems as described above for PIN diodes, such as uncounted x-ray photons resulting from production of electrons in the entrance window 115 and background signal due to production of electrons in the junction layer 116.